Re: [PATCH v1 0/5] s390: page_mapcount(), page_has_private() and PG_arch_1

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On 05.04.24 05:42, Matthew Wilcox wrote:
On Thu, Apr 04, 2024 at 06:36:37PM +0200, David Hildenbrand wrote:
On my journey to remove page_mapcount(), I got hooked up on other folio
cleanups that Willy most certainly will enjoy.

This series removes the s390x usage of:
* page_mapcount() [patches WIP]
* page_has_private() [have patches to remove it]

... and makes PG_arch_1 only be set on folio->flags (i.e., never on tail
pages of large folios).

Further, one "easy" fix upfront.

Looks like you didn't see:

Yes, I only skimmed linux-mm.

I think s390x certainly wants to handle PTE-mapped THP in that code, I think there are ways to trigger that, we're just mostly lucky that it doesn't happen in the common case.

But thinking about it, the current page_mapcount() based check could not possibly have worked for them and rejected any PTE-mapped THP.

So I can just base my changes on top of yours (we might want to get the first fix in ahead of time).

... unfortunately there is one other issue I spotted that I am not
tackling in this series, because I am not 100% sure what we want to
do: the usage of page_ref_freeze()/folio_ref_freeze() in
make_folio_secure() is unsafe. :(

In make_folio_secure(), we're holding the folio lock, the mmap lock and
the PT lock. So we are protected against concurrent fork(), zap, GUP,
swapin, migration ... The page_ref_freeze()/ folio_ref_freeze() should
also block concurrent GUP-fast very reliably.

But if the folio is mapped into multiple page tables, we could see
concurrent zapping of the folio, a pagecache folios could get mapped/
accessed concurrent, we could see fork() sharing the page in another
process, GUP ... trying to adjust the folio refcount while we froze it.
Very bad.

Hmmm.  Why is that not then a problem for, eg, splitting or migrating?
Is it because they unmap first and then try to freeze?

Yes, exactly. Using mapcount in combination with ref freezing is problematic. Except maybe for anonymous folios with mapcount=1, while holding a bunch of locks to stop anybody from stumbling over that.


David / dhildenb

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